1. Field of the Invention
The present invention is directed to an improved expansive cement and method of forming same. More specifically, the expansive cement of the present invention is formed by finely grinding an expansive clinker which includes a stable calcium alumino sulfate compound, and then blending an amount of the expansive clinker determined by the proportion of the calcium alumino sulfate compound therein with a separately ground cement clinker and gypsum to form an expansive cement composition.
2. Description of the Prior Art
Typical cements, such as a Portland cement, are generally comprised of discrete particles of a cement clinker and gypsum. Such cements, however, undergo an undesirable shrinkage as the cement sets and hardens. This shrinking is disadvantageous because, among other reasons, cracks tend to form in the hardened cement under conditions of restraint with the passage of time. Consequently, expansive clinkers are known in the art which can be added to the cement clinker for the purpose of producing a cement which will not shrink, that is, no net shrinkage occurs. See, for example, U.S. Pat. Nos. 3,155,526 and 3,251,701, the disclosures of which are hereby incorporated by reference. The addition of an expansive clinker in suitable proportion to the cement clinker can compensate for the normal shrinkage of the cement at least to some degree, if not entirely, as well as produce a net expansion.
Generally, the preparation of an expansive cement includes as the final step combining the discrete particles of the cement clinker with the discrete particles of the expansive clinker in a ball mill and then intergrinding the combined particles until the mixture has a suitable surface area as measured by ASTM C 204, also referred to as the Blaine number. However, when the expansive and cement clinkers are combined for intergrinding, control over the surface area of the expansive clinker is lost. For example, since the cement clinker is generally much harder than the expansive clinker, it is believed that intergrinding the expansive clinker with the cement clinker results in grinding the softer expansive clinker to generally much finer particle sizes than the cement clinker. Since an expansive cement can require up to about 50% more water to hydrate than a non-expansive cement, it is believed that the necessity for the additional water is caused at least in part by the smaller particle size of the expansive clinker. In an attempt to counteract this need for additional water, the prevailing tendency in the industry has been to more coarsely grind the expansive clinker by grinding the combined cement and expansive clinkers more coarsely. Moreover, since an inverse relationship exists between the amount of water added to hydrate a cement and the cohesive strength of the hardened product, the cohesive strength of an expansive cement has generally not been as great as non-expansive cements. This difference in cohesive strengths is undesirable.
In addition, forming the expansive cement by intergrinding the cement clinker and the expansive clinker generally requires at least about 12% by weight of the cement composition to be the expansive clinker, and oftentimes the expansive clinker comprises as much as 30-40% by weight of the cement composition, depending upon the proportion of the calcium alumino sulfate compound in the expansive clinker, in order to obtain a hardened product having the degree of expansion desired as well as the required strength. The amount of the expansive clinker added to form an expansive cement is of concern since the expansive clinker is generally much more expensive than the cement clinker. Consequently, the addition of lesser amounts of the expansive clinker will materially reduce the cost of an expansive cement.